The present invention relates to an apparatus and method for detecting knocking of an internal-combustion engine.
Conventionally, an occurrence of knocking is detected based on the output of a vibration sensor provided in an engine.
According to Japanese Patent Application Publication No. 2002-221074, a vibration sensor is provided in a cylinder of an engine. A wavelet transform is applied to the output of the vibration sensor to determine a vibration frequency characteristic. A combustion failure such as knocking or the like is detected by comparing the vibration frequency characteristic with a predetermined value.
Referring to FIG. 20(a), an example of the output of a vibration sensor provided in a cylinder of an engine is shown. The output of the vibration sensor varies due to an occurrence of knocking in regions 101 and 102. In regions 103 and 104, the output of the vibration sensor varies due to a noise generated when an intake/exhaust valve closes (referred to as a “seating noise”). Thus, the seating noise may be contained in the output of the vibration sensor. Such a seating noise may be erroneously determined as an occurrence of knocking.
FIG. 20(b) shows a waveform of a signal that is obtained by applying the wavelet transform to the output of the vibration sensor shown in FIG. 20(a) in accordance with the technique described in the above patent application publication. Frequency components corresponding to knocking (from 4.5 to 9 kHz) are extracted from the output of the vibration sensor by the wavelet transform. An impulse-like signal such as a seating noise has an influence on a wide range of frequencies. The influence of the seating noise appears in the signal obtained by the wavelet transform as shown in regions 105 and 106. Thus, even when the frequency components corresponding to knocking are extracted, there is still a possibility that an impulse-like noise such as a seating noise is erroneously determined as knocking.
In order to avoid such erroneous determination, a technique for setting a time period for detecting knocking for the output of the vibration sensor can be considered. However, if a longer detection period is set, an influence of various noises may appear in the detection period. If a shorter detection period is set, there is a possibility that knocking cannot be detected from the detection period when timing of knocking occurrence varies for some reason. For example, when sludge is built up in the engine, the timing of knocking occurrence may change. When a compression ratio of the engine changes, timing at which the pressure within a cylinder of the engine reaches the peak changes, which may displace the timing of knocking occurrence.
Thus, there is a need for an apparatus and a method that can locate an optimum period during which knocking occurs so as to detect the knocking more accurately.